The present invention relates to an airbag apparatus that inflates an airbag to protect an occupant when an impact is applied to a vehicle.
A side airbag apparatus having an inflator and an airbag is effective as an apparatus that protect an occupant when an impact is applied to a vehicle from a side due to a side collision. In such a side airbag apparatus, when an impact is applied from the side of a body side portion of the vehicle, the inflator discharges inflation gas into the airbag. The discharged inflation gas inflates and deploys the airbag forward from the backrest into a narrow space between the occupant seated in the vehicle seat and the body side portion. This reduces the impact that is transmitted from the side to the occupant through the body side portion.
The impact resistance of the sides of the human body varies from part to part. For example, the lumbar region has a better impact resistance than the thorax. In the case of a side airbag apparatus that protects an occupant in an area ranging from the lumbar region to the thorax, it is preferable that an inflated and deployed airbag protect a region of a lower impact resistance (the thorax) more gently than a region of a higher impact resistance (lumber region).
Japanese Laid-Open Patent Publication Nos. 2004-243976, 2006-8017, and 2008-7104 each disclose an airbag apparatus that is designed with the above described impact resistance of the human body. In each of the airbag apparatus disclosed in these publications, the interior of the airbag is divided into a plurality of inflation sections (chambers) by partitioning means such as seams and tethers. By varying the flow rate of inflation gas supplied from the inflator to each inflation section, the internal pressure of each inflation section has a value suitable for the impact resistance of the corresponding part of the side of the occupant's body. Further, a vent hole is formed in an inflation section of the airbag that protects a low impact resistance region of the occupant (for example, the thorax). Some of the inflation gas in the inflation section is discharged to the outside through the vent hole, so that the internal pressure of the inflation section is adjusted so as to be prevented from being excessively raised. Therefore, when the inflation section of which the internal pressure has been raised by inflation receives pressing force by the occupant due to restraint, some of the inflation gas is discharged through the vent hole. This inhibits the further increase of the internal pressure of the inflation section, so that a low impact resistance region of the occupant (for example, the thorax) is protected by the inflation section having an appropriate hardness.
However, in the airbag apparatuses of the above publications, the vent hole is always open in the inflation section. Thus, inflation gas starts being discharged through the vent hole at a relatively early stage after the start of inflation and deployment of the inflation section. The primary objective of the vent hole is to discharge excessive inflation gas to the outside of the airbag that would otherwise excessively increase the internal pressure of the inflation section. That is, the vent hole is provided for discharging inflation gas when the internal pressure of the inflation section is likely to further increase on the assumption that the internal pressure of the inflation section has already been increased to a relatively high level. However, in the above described structure in which the vent hole is always open, part of the inflation gas starts being discharged through the vent hole without being used for inflation and deployment before the internal pressure of the inflation section becomes sufficiently high. As a result, although each of the airbag apparatuses according to the above publications is capable of inhibiting excessive increase of the internal pressure of the inflation section, it takes relatively long time for the inflation section to be inflated and deployed since the inflation gas starts being discharged at a relatively early stage. Also, to achieve the required performance (quick inflation of the airbag and prevention of excessive internal pressure of the inflation section), this configuration requires additional amount of inflation gas that is not effectively utilized for restraining the occupant. As described above, the above described conventional airbag apparatuses still have room for improvement in the deployment performance of the airbag and the protection performance.